Commercial communication satellites typically have receive antenna coverage patterns that encompass very large areas. Uplink interference signals, generated anywhere within the coverage area, can disrupt communication services. Sometimes, the cause of the interference can be determined merely by analyzing the modulation of the interference signal. However, in most cases, the only way to identify the source of the interference is to determine its physical location.
One method of locating the interference source is to physically move the satellite antenna and measure the effect on the interference signal. Knowing the satellite antenna pattern and its movement, the interference signal level variation allows an attempt at matching the signal behavior with a specific segment of the antenna pattern.
There are several disadvantages associated with this method. For example, physical movement of the satellite's antenna could potentially disrupt normal communications. Additionally, the interference source may be located at a portion of the antenna pattern where the signal level may not be sufficient, i.e. a flat portion of the antenna pattern. It is also possible that the behavior of the interference signal level may not be unique to a single portion of the pattern, making location virtually impossible.
Another location technique depends on the reception of the interference signal at two separated satellites as described in U.S. Pat. No. 5,008,679 to Effland et al. At least two satellite receivers receive an interfering signal from an unknown transmitter and retransmit the signals to a receiving station. The receiving station receives and processes the retransmitted interfering signals through cross-correlation to determine the location of the unknown transmitter.
Two separate co-located earth terminals receive the same interference signal from separate satellites. The time difference between the two signals is measured using some modulation characteristic of the interference as a time marker. If the locations of the satellites are known with a sufficient degree of accuracy, a line of position on the surface of the earth can be computed. A second, intersecting line of position can be generated using a third satellite in combination with one of the original two satellites. The accuracy of the position fix is dependent on the orientation of the satellite to satellite baselines for each measurement.
Obviously, a drawback to the approach mentioned above is that two satellites are required to determine a single line or position. A third satellite, preferably not in the same plane as the first two satellites, is required to determine a position fix. Another drawback is that the interference signal must be modulated with a waveform in order to provide an unambiguous time tag.
An addition to this technique, which is described in U.S. Pat. No. 5,570,096 to Knight et al., provides for the determination of the second line of position utilizing measurements of relative Doppler frequency shift on the signals received from the two satellites. This system significantly increases the complexity of the measurement and calculation. The approach still requires reception via at least two satellites and the interference must be modulated to permit time difference measurements.